The present invention generally relates to perpendicular magnetic recording mediums and manufacturing methods thereof, and more particularly to a perpendicular magnetic recording medium having satisfactory perpendicular magnetic recording and reproducing characteristics and a manufacturing method thereof.
Generally, when recording and reproducing a signal on and from a magnetic recording medium by use of a magnetic head, the magnetic head magnetizes a magnetic layer of the magnetic recording medium in a longitudinal direction of the magnetic recording medium (that is, in an in-plane direction) at the time of the recording and picks up the recording at the time of the reproduction. However, according to such a longitudinal magnetic recording system, it is known that the demagnetization field becomes high as the recording density increases and the demagnetization field introduces undesirable effects to the high density magnetic recording. Hence, in order to eliminate the undesirable effects of the demagnetization, a perpendicular magnetic recording system has been proposed in which the magnetic head magnetizes the magnetic layer of the magnetic recording medium in a direction perpendicular to the magnetic layer. According to the perpendicular magnetic recording system, the demagnetization field becomes low as the magnetic recording density increases, and theoretically, it is possible to realize a satisfactory high density magnetic recording in which there is no decrease in the remanent magnetization.
As a conventional perpendicular magnetic recording medium which is used in the perpendicular magnetic recording system, there is a perpendicular magnetic recording medium having a cobalt-chromium (Co-Cr) film formed on a base film by a sputtering process. As is well known, the Co-Cr film is extremely suited for use in the perpendicular magnetic recording medium because the Co-Cr film has a relatively high saturation magnetization (Ms) and favors magnetization in a direction perpendicular to the Co-Cr film (that is, the coercivity in the direction perpendicular to the Co-Cr film is large and the axis of easy magnetization is perpendicular to the Co-Cr film).
However, when perpendicular magnetic head performs the perpendicular magnetic recording and reproduction with respect to the perpendicular magnetic recording medium having the sputtered Co-Cr film, it is impossible to concentrate the magnetic flux at a predetermined magnetic recording position on the perpendicular magnetic recording medium, and there is a disadvantage in that it is impossible to obtain a strong magnetization which is in the direction perpendicular to the Co-Cr film and does not spread in the longitudinal direction of the perpendicular magnetic recording medium. In other words, when a ring core head is used to perform the recording on the Co-Cr film of the perpendicular magnetic recording medium, the magnetization direction easily deviates in the longitudinal direction of the perpendicular magnetic recording medium since the magnetic field generated by the ring core head includes considerable components in the in-plane direction. Accordingly, in order to maintain the magnetization direction in the perpendicular direction, the perpendicular magnetic recording medium must have a high perpendicular anisotropic magnetic field and have a saturation magnetization which is suppressed to a certain extent. However, the Co-Cr film does not have such characteristics, and there is a disadvantage in that it is impossible to perform a satisfactory perpendicular magnetic recording by the perpendicular magnetic head with the exception of the perpendicular magnetic head of the type having an auxiliary magnetic pole opposing a main magnetic pole. In addition, the coercivity in the perpendicular direction must be large in order to obtain a high reproduced output from the perpendicular magnetic recording medium having the Co-Cr film. On the other hand, it is desirable to make the thickness of the perpendicular magnetic recording medium large in order to decrease the demagnetization field, but the perpendicular magnetic recording medium will not make contact with the perpendicular magnetic head in a satisfactory state when the thickness of the perpendicular magnetic recording medium is large because the perpendicular magnetic recording medium will lose its flexibility and become rigid. In this case, there are disadvantages in that the rigid perpendicular magnetic recording medium is easily damaged and undesirable effects are introduced to the perpendicular magnetic head, and it is impossible to perform a satisfactory perpendicular magnetic recording and reproduction.
Accordingly, a perpendicular magnetic recording medium having a double film construction has been proposed. According to this perpendicular magnetic recording medium, a film having a high permeability, that is, a film having a low coercivity such as a nickel-ion (Ni-Fe) film, is formed between the Co-Cr film and the base film. The magnetic flux which is spread within the high permeability film is concentrated toward the magnetic pole of the perpendicular magnetic head at a predetermined magnetic recording position in order to obtain a strong magnetization which is in the perpendicular direction and does not spread in the longitudinal direction of the perpendicular magnetic recording medium. However, in the case of the perpendicular magnetic recording medium having the double film construction, the coercivity of the high permeability film is extremely small compared to the coercivity of the Co-Cr film, and there is a disadvantage in that Barkhausen noise is generated. For example, the coercivity of the Co-Cr film is over 700 Oe, and the coercivity of the high permeability film is under 10 Oe. Further, in order to produce the perpendicular magnetic recording medium having the double film construction, an amorphous (ion-nickel) Fe-Ni alloy or the like is formed on the base film by a sputtering process under a predetermined sputtering condition suited for forming the high permeability film, and Co-Cr is thereafter formed on the high permeability film by a sputtering process under a certain sputtering condition suited for forming the Co-Cr film. As a result, the sputtering condition under which the sputtering process is performed and the target must be changed for the formation of ech film, and the sputtering processes cannot be performed continuously. Therefore, there are disadvantages in that the processes of manufacturing the perpendicular magnetic recording medium are complex and unsuited for mass production.